A. Field of the Invention
This invention relates to the field of art of photovoltaic solar cells and especially tandem photovoltaic solar cells.
B. Background Art
A photovoltaic solar cell, often referred to as a solar cell, is a semiconductor device which converts light energy into electrical energy. Solar cells are useful for producing electrical energy in a variety of applications and environments. Among these, solar cells are particularly advantageous energy sources for use in space applications such as orbiting satellites, space vehicles and the like.
A consideration which critically affects the use of solar cells is energy conversion efficiency, that is, the ratio of light energy per unit time incident on a solar cell to the electrical energy per unit time produced by the cell. In general, the higher the conversion efficiency, the more useful the solar cell. Cost, ease of manufacture and reliability are also important.
An approach to achieving higher efficiency is described in U.S. Pat. No. 2,949,498 to E. D. Jackson. The Jackson patent discloses a multilayer solar cell with each layer having a p-n junction and being composed of a semiconductor material having a different energy gap than the other layers. The layers in the cell of Jackson are arranged to receive light in descending order of their energy gaps. Such cells are known in the art as tandem cells.
Tandem cells utilizing a silicon cell for the low energy gap cell and a III-V compound semiconductor cell for the upper, high energy gap cell are described by J. C. C. Fan et al. in the Conference Record of the 16th I.E.E.E. Photovoltaic Specialists Conference, p. 692 (1982) published by the Institute of Electrical and Electronics Engineers, New York, N.Y. According to Fan et al., the optimum energy gap, or band gap, for a top solar cell used in tandem with a silicon bottom cell having a band gap of 1.1 eV lies between 1.77 and 2.09 eV. This combination of band gaps relates to a theoretical efficiency of 32.9% at AMO and 27.degree. C. for a four terminal tandem cell.
In an article by J. C. C. Fan et al. in Photovoltaics for Solar Energy Applications II, D. Adler, Editor, Proceedings of SPIE, Volume 407, pp. 73-87 (1983) published by the International Society of Optical Engineering, a two cell tandem structure is dislosed with a silicon bottom cell and Ga.sub.1-x Al.sub.x As or GaAs.sub.1-x P.sub.x with x about 0.3 for the top cell.
U.S. Pat. No. 4,278,474 to A. E. Blakeslee et al. describes a monolithic series connected tandem cell having silicon for the bottom cell and gallium arsenide phosphide for the top cell with an intervening super lattice structure to block propogation of dislocation defects. Such a cell is difficult to fabricate and is limited to a two terminal configuration.
U.S. Pat. No. 4,338,480 to G. A. Antypas et al. discloses an AlGaAs cell stacked on top of a silicon cell. The AlGaAs cell of Antypas et al. is disclosed as being grown on a GaAs substrate. However, GaAs has an energy gap of about 1.43 eV and would prevent a significant amount of the light energy required for tandem cell operation from reaching the silicon bottom cell.
Furthermore, Ga.sub.1-x Al.sub.x As has proven not to be highly transparent to photons whose energy is less than the band gap. Another disadvantage of Ga.sub.1-x Al.sub.x As is that it is liable to oxidation and reaction with moisture. Still another disadvantage of Ga.sub.1-x Al.sub.x As is the difficulty in making low resistance ohmic electrical contact. Accordingly, advantages of tandem solar cells utilizing III-V alloy top cells have yet to be realized.
An object of this invention is a gallium arsenide phosphide top solar cell fabricated on a transparent gallium phosphide substrate and placed in electrical series with a silicon solar cell for a two terminal solar cell or wired separately for a four terminal tandem solar cell having high conversion efficiency.
Another object of this invention is to provide a gallium arsenide phosphide top cell having a transition layer between the gallium arsenide phosphide and the transparent gallium phosphide substrate in order to minimize dislocations.
Another object of this invention is to provide a gallium arsenide phosphide top cell having a gallium phosphide cap layer to minimize surface recombination.